1. Field of the Invention
The present invention relates to an apparatus for refining molten steel in a secondary refining process to manufacture ultra low carbon steel and a method for refining molten steel by utilizing the apparatus.
2. Description of the Prior Art
Generally, when an ultra low carbon steel with a carbon content of 70 ppm or less is manufactured, an RH vacuum degassing apparatus 100 (to be called "RH" below) of FIG. 1 is used. In the method using this apparatus, when molten steel which is tapped from a converter (not shown in the drawings) without killing during tapping arrives at the RH, firstly argon (Ar) gas is injected from a circulation gas supplying device 130, and at the same time, a snorkel 120 is dipped into a molten steel M which is contained within a ladle 140. At the same time, a vacuum pump 125 is activated to reduce the internal pressure of a vessel 110 to several Torr or several scores of Torr. Under this condition, molten steel M in the ladle 140 ascends into the interior of the vacuum vessel owing to the pressure difference between the vessel 110 and the atmosphere. At the same time, a decarburizing reaction occurs on the surface of molten steel M as shown in Equation (1) below. As the decarburizing reaction proceeds, the carbon content within molten steel M is decreased, and after elapsing of 15 to 25 minutes, the carbon content within molten steel M reaches 70 to 25 ppm. EQU [C]+[O]=CO(g) (1)
That is, in the case where molten steel is refined by using the RH 100 of FIG. 1, a period of 15 minutes or more is required in reducing the carbon content to 70 ppm or less. Further, the temperature of molten steel is lowered by 1.5.degree. C. per minute during the decarburization process, which is also a problem.
Meanwhile, Japanese Patent Application Laid-open Nos. Sho-52-88215 and Sho-52-89513 disclose molten steel refining apparatuses for manufacturing ultra low carbon steel. These apparatuses are constituted as follows. As shown in FIG. 2, a lance nozzle 150 is installed on the ceiling of the RH vessel 110, for injecting gaseous oxygen so as to shorten the decarburization period when producing ultra low carbon steel. Thus, during the decarburization of molten steel, gaseous oxygen is injected at high speed through the lance nozzle 150 onto the surface of molten steel within the vessel 110.
Further, Japanese Patent Application Laid-open Nos. Hei-4-289113, Hei-4-289114 and Hei-4-308029 disclose other apparatuses. These apparatuses are constituted as follows. As shown in FIG. 3, a height adjustable lance nozzle 160 is installed on the ceiling of the RH vessel 110, for injecting argon gas. During the decarburization of molten steel M for manufacturing ultra low carbon steel, argon gas is injected through the lance nozzle 160 onto the surface of molten steel M. When the carbon content of molten steel reaches 50 ppm, the lance nozzle 160 is dipped into molten steel M within the vessel 110 so as to inject argon gas into molten steel M, thereby manufacturing ultra low carbon steel.
In the apparatuses of FIGS. 2 and 3, the lance nozzles 150 and 160 are made of copper. In the case where these apparatuses are used for carrying out the decarburization, argon and oxygen are injected onto the surface of molten steel M, so that the decarburization speed for ultra low carbon steel is promoted, and to minimize the temperature decrease within the vessel 110.
However, in the case where the apparatuses of FIGS. 2 and 3 are used for carrying out the decarburization, the internal temperature of the vacuum vessel 110 is raised to 800 to 1200.degree. C., with the result that the copper lance is liable to be damaged or partially melted. If the cooling water leaks, the cooling water intensely reacts with molten steel at 1600.degree. C., creating the possibility that the vacuum vessel may explode.
Japanese Patent Application Laid-open No. Sho-64-217 discloses another apparatus. In this apparatus, two straight tubes are installed on the side wall of the RH vessel, and carbon monoxide is injected through the straight tubes (single layer tubes) during refining, while oxygen is injected through a lance which is installed on the ceiling of the RH. Thus, the combustion of carbon monoxide decreases the temperature drop of the molten steel during refining.
In the case where carbon monoxide is injected through the straight tubes as in the above method, the combustion of carbon monoxide produces a flame jet, the shape of which is shown in FIG. 14A. In this method, carbon monoxide reacts with oxygen which is injected from the ceiling, thereby preventing an excessive temperature drop in the molten steel. However, the promotion of decarburization reaction is difficult, and the cooling capability of the straight tubes of single layer is deteriorated. Therefore, when the use period is extended, the straight tubes are apt to be melted by radiation heat of molten steel, and the surrounding refractory is melt-damaged.
Japanese Patent Application Laid-open No. Sho-63-19216 discloses another apparatus. In this apparatus, a plurality of single layer straight tubes are installed with different heights on the side wall of the RH vessel. Thus, during the decarburization of molten steel, oxygen is injected onto the surface of molten steel within the RH vessel.
Since the nozzle for injecting oxygen is attached to the straight tube, the oxygen stream does not form a jet stream, but forms the oval shape of FIG. 14A. Injected oxygen gas is used to supply oxygen into molten steel.
In this method, however, since the injected oxygen gas does not form ajet stream, a cavity on molten steel surface cannot be formed. Therefore, the area in which the decarburization occurs cannot be expanded and the problem that the decarburization cannot be promoted occurred.
Further, in this method, since a plurality of the straight tubes are installed on the side wall of the RH vessel, the evacuating capability for vacuum is greatly deteriorated, and therefore, its practicality is questionable. Further, as the use period elapses, the single layer straight tubes undergo lowering of the cooling capability, and therefore, melting loss occurs. Further, melting loss occurs in the surrounding refractory materials, and therefore, the life expectancy of the RH vacuum vessel is significantly shortened. Therefore, the apparatus is economically disadvantageous.